This invention refers to a device for automatically positioning a sheet on a platen of an office machine, including a guide element to guide a sheet towards the writing line and an element to guide the sheet over said writing line. The device is, according to the invention, particularly fit to be used in typewriting machines, accounting machines, terminals and similar machines for office use, in which the printing unit is movable with respect to the writing platen, which can only turn with respect to the fixed sides, without translating. In this kind of machine, to let the printing unit freely translate along the writing line from a platen end to another, the driving elements of the sheet must be placed only upon and under said writing line. Normally, to introduce a sheet in the machine and to position it correctly on the platen, the operator must manually operate the platen to drive the sheet towards the upper driving elements and to prevent interference with the printing unit which, in this circumstance, must be removed from the platen. The technical problem that this invention intends to solve is the realization of a device which permits the automatic positioning of a sheet with respect to a writing platen of an office machine which is easy, reliable and cheap, and which does not require previous removal of the printing unit from the writing platen.